Studies on the structure and function of the teleost pseudobranch
Four main types of pseudobranch were distinguished on the basis of their epithelial covering. The terms 'free', ‘semi-free’, 'covered' and 'buried' are used to describe these types. All pseudobranchs possess specific ‘pseudobranch type’ cells, characterised by an orderly arrangement of tubules around closely packed mitochondria. They also contain vacuoles which may play a role in osmoreception by causing changes in the size and shape of the cells. Another specialised cell type, similar to the gill 'chloride' cell was found in 'free' and 'semi-free' pseudobranchs of salt water fish. They are associated with smaller 'accessory' cells from which they are separated by 'leaky' junctions which may provide a structural basis for the proposed ion secretory nature of 'chloride type' cells. Ultrastructural changes in 'chloride type' and 'pseudobranch type' cells were noted under osmotic stress but the cells still remained distinguishable from each other, giving no reason to suggest that they were different forms of the same cell. The two other main cell types found in the pseudobranch epithelium were mucous and rodlet cells. The epithelial surface possesses numerous microridges which are thought to aid anchorage of mucous. The vascular system of the pseudobranch shows close similarities to that of the gill and contains a well developed arterio-venous pathway as well as an arterio-arterial system. Arterio-venous anastomoses were found between the efferent filament artery and the central venous sinus of the bass pseudobranch. The pseudobranch innervation is extremely complex. Morphological and denervation studies suggest an autonomic innervation and physiological evidence indicates the presence of at least two and possibly four types of receptor. The results of this study indicate that the pseudobranch has a number of inter-related functions associated with the development of specific cell types and a complex innervation linked directly or indirectly to the vascular system.